Residual force enhancement is not altered while force depression is amplified at the cellular level in old age
Bibliographic record
Abstract
Residual force enhancement (rFE) and residual force depression (rFD) are history-dependent properties of muscle which refer to increased and decreased isometric force following a lengthening or shortening contraction, respectively. The history dependence of force is greater in older than in younger human adults when assessed at the joint level. However, it is unclear whether this amplification of the history dependence of force in old age is owing to cellular mechanisms or is a consequence of age-related remodelling of muscle architecture. Single muscle fibres from the psoas major of old and young F344BN rats were dissected and chemically permeabilized. Single muscle fibres were mounted between a force transducer and length controller, then maximally activated (pCa 4.5). To assess rFD, fibres were actively shortened from 3.1 to 2.5 µm at both a slow (0.15 Lo s-1) and fast (0.6 Lo s-1) speed, with a fixed-end isometric reference contraction at 2.5 µm. To assess rFE, fibres were activated and stretched at 0.3 Lo s-1 from a sarcomere length of 2.2 to 2.5 µm, and 2.7 to 3.0 µm, and compared with fixed-end isometric reference contractions at 2.5 and 3.0 µm, respectively. Isometric force (2.5 µm) was ∼19% lower in muscle fibres from old as compared with young rats (P<0.001). Upon normalizing to fibre cross-sectional area, there was no age-related difference in specific force (P>0.05). rFD was ∼33% greater in muscle fibres from old as compared with young rats (P<0.05), while rFE did not differ between groups (P>0.05). rFD is amplified in old age at the cellular level, while rFE appears to be unchanged; thus, previously reported age-related modification of rFE occurs upstream from the cellular level.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".